Document how VOL connectors should format object tokens

HDF5's expectations for how the bytes of object tokens should be formatted need to be documented. Specifically, what needs to be documented is:

- Whether unused bytes of an object token (when the size of a token is less than `H5O_MAX_TOKEN_SIZE`) can be left uninitialized or if they should be set to a specific value or any value
- Whether there is a 1 to 1 mapping between an object and a particular object token byte stream or if multiple different object token byte streams can point to the same object
- The significance of the `token_size` field, as returned by the `H5VL_FILE_GET_CONT_INFO` VOL operation